If you are placing the battery on the inside firewall (I am for the second battery) consider using this: ETX900 Vent I have been a long time user of Lithium based batteries and have seen one very small battery emit a lot of noxious fumes.
RV-14A QB building, Lycoming 390 Thunderbolt arrived July 2019, Garmin avionics, Vertical Power, EFII-32 Ignition and Fuel, hope to be flying Spring of 2020 !! Dues paid+ of course !!